Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic signals (sound waves) and convert received acoustic waves to electrical signals via inverse and direct piezoelectric effect. Acoustic transducers generally include acoustic resonators, such as surface acoustic wave (SAW) resonators and bulk acoustic wave (BAW) resonators, and may be used in a wide variety of electronic applications, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other portable communications devices. BAW resonators include an acoustic or resonator stack disposed over an acoustic reflector. For example, BAW resonators include thin film bulk acoustic resonators (FBARs), which include resonator stacks formed over a substrate cavity, which functions as the acoustic reflector, and solidly mounted resonators (SMRs), which include resonator stacks formed over alternating stacked layers of low acoustic impedance and high acoustic impedance materials (e.g., an Bragg mirror). The BAW resonators may be used for electrical filters and voltage transformers, for example.
Generally, an acoustic resonator has a layer of piezoelectric material between two conductive plates (electrodes), which may be formed on a thin membrane. The piezoelectric material may be a thin film of various materials, such as aluminum nitride (AlN), zinc oxide (ZnO), or lead zirconate titanate (PZT), for example. Thin films made of AlN are advantageous since they generally maintain piezoelectric properties at a high temperature (e.g., above 400° C.). The acoustic stack of a BAW resonator comprises a first electrode, a piezoelectric layer disposed over the first electrode, and a second electrode disposed over the piezoelectric layer. The acoustic stack is disposed over the acoustic reflector. The series resonance frequency (Fs) of the BAW resonator is the frequency at which the dipole vibration in the piezoelectric layer of the BAW resonator is in phase with the applied electric field. On a Smith Chart, the series resonance frequency (Fs) is the frequency at which the Q circle crosses the horizontal axis. As is known, the series resonance frequency (Fs) is governed by, inter alia, the total thickness of the layers of the acoustic stack. As can be appreciated, as the resonance frequency increases, the total thickness of the acoustic stack decreases. Moreover, the bandwidth of the BAW resonator determines the thickness of the piezoelectric layer. Specifically, for a desired bandwidth a certain electromechanical coupling coefficient (kt2) is required to meet that particular bandwidth requirement. The kt2 of a BAW resonator is influenced by several factors, such as the dimensions (e.g., thickness), composition, and structural properties of the piezoelectric material and electrodes. Generally, for a particular piezoelectric material, a greater kt2 requires a greater thickness of piezoelectric material. As such, once the bandwidth is determined, the kt2 is set, and the thickness of the piezoelectric layer of the BAW resonator is fixed. Accordingly, if a higher resonance frequency for a particular BAW resonator is desired, any reduction in thickness of the layers in the acoustic stack cannot be made in the piezoelectric layer, but rather must be made by reducing the thickness of the electrodes.
While reducing the thickness of the electrodes of the acoustic stack provides an increase in the resonance frequency of the BAW resonator, this reduction in the thickness of the electrodes comes at the expense of performance of the BAW resonator. For example, reduced electrode thickness results in a higher sheet resistance in the electrodes of the acoustic stack. The higher sheet resistance results in a higher series resistance (Rs) of the BAW resonator and an undesired lower quality factor around series resonance frequency Fs (Qs). Moreover, as electrode thickness decreases, the acoustic stack becomes less favorable for high parallel resistance (Rp) and as a result the quality factor around parallel resonance frequency Fp (Qp) is undesirably reduced.
What is needed, therefore, is a BAW resonator that overcomes at least the shortcomings of known BAW resonators described above.